vfs: Call ->sync_fs() even if s_dirt is 0 (version 4)

sync_filesystems() has a condition that if wait == 0 and s_dirt == 0, then
->sync_fs() isn't called. This does not really make much sence since s_dirt is
generally used by a filesystem to mean that ->write_super() needs to be called.
But ->sync_fs() does different things. I even suspect that some filesystems
(btrfs?) sets s_dirt just to fool this logic.
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/super.c

@@ -510,7 +510,7 @@ restart:
 		sb->s_count++;
 		spin_unlock(&sb_lock);
 		down_read(&sb->s_umount);
-		if (sb->s_root && (wait || sb->s_dirt))
+		if (sb->s_root)
 			sb->s_op->sync_fs(sb, wait);
 		up_read(&sb->s_umount);
 		/* restart only when sb is no longer on the list */